The present invention relates to a semiconductor laser device for optical communication. A single mode oscillation semiconductor laser device according to the present invention is useful for dense wavelength division multiplexing transmission.
With the spread and development of the Internet, the volume of information transmitted to each household is increased. The demand for greater volume has accelerated development of the dense wavelength division multiplexing (hereinafter abbreviated to DWDM) transmission system that enables high-volume transmission. In this system, the number of wavelengths of light transmitted through a single optical fiber is increased for higher transmission volume. On the other hand, an increase in the number of wavelengths contributes to narrowing of a channel spacing. This is because there is a limit to loss characteristics of fibers applicable to optical communication, and there is accordingly a limit to a transmission wavelength band for practical use. The narrowing of a channel spacing tends to cause crosstalk between neighboring channels. In order to avoid such crosstalk, required accuracy of transmission wavelengths is made stricter. For example, when transmission wavelengths are arranged in a wavelength band of 60 nm at a spacing of 0.8 nm, information for 64 channels or 80 channels can be transmitted. In this case, the stability of the wavelengths required in transmission is xc2x10.01 nm. Thus, the required yield of wavelengths of semiconductor laser devices serving as optical sources is now extremely high. Therefore, transmitting optical sources that meet such wavelength specifications are fabricated at extremely high cost.
On the other hand, in a future DWDM system, the number of transmitting optical sources required will be further increased. Accordingly, it is desired that cost for a single transmission channel be further reduced. Thus, with regard to transmitting optical sources, it is necessary to realize a semiconductor laser array in which low-cost, strongly-built, and compact semiconductor lasers with different oscillation wavelengths from each other are integrated.
A transmitting optical source used for DWDM has a wavelength selecting function for providing a single wavelength. For example, a distributed feedback laser (hereinafter abbreviated to a DFB laser), a typical example of a single mode oscillation laser, has a diffraction grating structure shaped like a saw blade in the vicinity of its optical waveguide layer. The periodicity of refractive indexes of the diffraction grating structure has an optical effect on light propagating in the waveguide. When this optical effect is specifically described, the oscillation wavelength (xcex) of a single mode semiconductor laser is determined by the following equation:
xcex=(2xc3x97nxcex9)/m
where n is the equivalent refractive index of transmitting waveguide structures, A is the period of the grating, and m is a degree. It is understood from this equation that in order to control oscillation wavelength accurately, it is desired to suppress variations in the equivalent refractive index (n). In order to achieve this, it is desirable to be able to control the equivalent refractive index (n) readily and accurately in device fabrication.
The equivalent refractive index (n) in the above equation is determined not only by the refractive index possessed by the material of the active layer where light propagates, but also by the shape and dimensions of the active layer and the refractive index of a structure around the active layer. Therefore, in order to control oscillation wavelength accurately, it is also necessary to control the shape and dimensions of the active layer of a semiconductor laser device and suppress variations in the refractive index of the structure around the active layer.
Basic structures of conventional semiconductor lasers are roughly divided into a gain-guide type structure and a refractive index waveguide type structure. Atypical example of a gain-guide type semiconductor laser is a ridge waveguide semiconductor laser. In fabricating a ridge type semiconductor laser, a semiconductor laminate structure that serves as a base is formed by a single crystal growth process. Thereafter, while leaving a light emitting region, an upper cladding layer, which is a region that sandwiches the light emitting region, is etched, and is then buried in a polyimide resin.
On the other hand, a refractive index waveguide type semiconductor laser, which is typified by a buried heterostructure laser device, has a buried heterostructure in which only the waveguiding region for light in the semiconductor laminate is made to remain, and the other regions are buried in substrate material. In a process of fabricating this single mode oscillation semiconductor laser, a current blocking layer is formed by a regrowth process step after etching.
The process of fabricating the structure of the above-mentioned ridge waveguide semiconductor laser is simple, and therefore, in the case of Fabry-Perot lasers, the yield of their fabrication is very high. The side walls of the ridge shape of the upper cladding layer are covered with polyimide. Therefore, a relative refractive index difference between the active layer formed by semiconductor material and the ridge sides formed by non-semiconductor material is very large. Thus, reflecting variations in the equivalent refractive index (n) caused by variations in active region width on the ridge side, variations in the oscillation wavelength of the single mode oscillation laser become significant.
On the other hand, in the case of the buried heterostructure semiconductor laser, nonuniformity in the structure of its active region is significant, as compared with the ridge waveguide semiconductor laser. Thus, variations in the equivalent refractive index of the buried heterostructure semiconductor laser proper become significant, thereby making it difficult to control the oscillation wavelength accurately.
Moreover, DWDM transmitting optical sources present a problem other than that of the precision of oscillation wavelength in device fabrication. The problem is a drift of lasing wavelength resulting from secular changes of a transmitting optical source itself mounted in a system. In order to deal with this problem, development of wavelength variable lasers to be used as transmitting optical sources has been conducted. The wavelength variable laser is a single mode oscillation semiconductor laser mounted with a heater, so that its oscillation wavelength is changed by heating the active layer. The heating of the active layer, however, impairs characteristics of the semiconductor laser. Therefore, in the development of DWDM transmitting optical sources, it is essential to fabricate a single mode oscillation semiconductor laser that can oscillate at a required wavelength with accuracy and to improve temperature characteristics of the semiconductor laser itself for use as a wavelength variable laser. Factors in the impairment of temperature characteristics of conventional semiconductor lasers will be described in the following. In the above-mentioned ridge waveguide semiconductor laser, carriers injected into the active layer are diffused laterally as the temperature of the active layer rises. Therefore, it becomes necessary to inject an excess amount of carriers to compensate for a decrease in gain resulting from the temperature rise.
In the case of the buried heterostructure semiconductor laser, there is a decrease in electric resistance in the vicinity of an interface between the etched active layer and the semiconductor material for burying the active layer. Thus, as the temperature of the active layer rises, carriers flow out through this region, and therefore are not effectively injected into the active layer.
In view of the technical background described above, it is a first object of the present invention to provide semiconductor laser devices and semiconductor laser array devices that can ensure high-precision oscillation wavelengths.
It is a second object of the present invention to provide semiconductor laser devices and semiconductor laser array devices that are less affected by the atmospheric temperature while ensuring high-precision oscillation wavelengths.
It is a third object of the present invention to provide semiconductor laser devices and semiconductor laser array devices that make it possible to achieve the first or second object described above and also ensure a certain yield level in fabrication.
According to the present invention, it is possible to provide single mode oscillation semiconductor laser devices and semiconductor laser array devices that have such characteristics as described above.
It is another object of the present invention to provide optical systems or optical fiber transmission systems that enable transmission at stable wavelengths.
Main points of the present invention will be described with reference to an example in FIG. 1. FIG. 1 is a perspective view of a semiconductor laser device according to the present invention. The figure shows two laser structures; a laser structure 1 on the left side is a schematic view of the semiconductor laser device and the other laser structure on the right side is a partially sectioned view of the semiconductor laser device for facilitating understanding of the device structure.
On a semiconductor substrate 11, a buffer layer 12 and a cladding layer 13 (also commonly referred to as an optical guide layer) on the substrate side are formed. On the cladding layer 13, an active layer region 4 and an upper cladding layer 21 are disposed. Thus, the active layer region 4 forms an optical waveguide. Generally, the active layer region 4 has a quantum-well structure. The quantum-well structures generally applied to semiconductor laser devices include, for example, a single quantum-well structure, a multiple quantum-well structure, a strained quantum-well structure, or a strain-compensated quantum-well structure. Such structures can be used according to the requirements of a semiconductor laser device to be employed. Incidentally, in this case, a strained quantum-well structure is adopted as a concrete example.
In order to ensure oscillation in single mode operation, it is particularly desirable to use a diffraction grating 10 for optical feedback. In addition to the DFB type laser of this example, in the present invention, a so-called DBR (Distributed Bragg Reflection) type laser employing a diffraction grating is desirable. Desirable arrangements of the diffraction grating will be shown in concrete structures illustrated in several embodiments to be described later. However, it is needless to say that a given diffraction grating for optical feedback may be disposed at a location on a layer or in a region that is not described in the present specification. In other words, it is sufficient if the structure enables DFB type or DBR type laser oscillation. Of course, regardless of the type of optical feedback means, the basic concept of the present invention of providing a first current blocking layer region and a second current blocking layer region, which will be described below, provides sufficient current blocking effects.
According to a representative aspect of the present invention, a first current blocking layer region 3 having the characteristics described below is provided so as to sandwich an active layer region of the semiconductor laser device. Of course, the first current blocking layer region is disposed on both sides of the optical resonator so as to intersect a traveling direction of light.
It may be said that a region corresponding to the first current blocking layer region 3 has an electric resistance different from that of the active layer region of the semiconductor laser device and has a refractive index higher than that of substrate material. The first current blocking layer 3 or the region having an electric resistance different from that of the active layer region of the semiconductor laser device and having a refractive index higher than that of substrate material is typically formed by implanting ions into a fundamental semiconductor laminate that forms the semiconductor laser device.
According to another aspect of the present invention, a second current blocking layer region 6 formed by semi-insulating semiconductor material is disposed on the first current blocking layer region 3. It is also an important aspect from a practical point of view that the second current blocking layer region 6 is formed in a self-aligned relation with the first current blocking layer region 3, and therefore the width of the second current blocking layer region 6 is thus determined. In a specific example of forming the second current blocking layer 6, the first current blocking layer region 3 is formed by ion implantation, and then a mask used for the ion implantation is utilized to selectively form the second current blocking layer region 6 by crystal growth.